In mobile communication networks, there is always a challenge to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the mobile communication network is deployed.
In national security and public safety (NSPS) scenarios, there is a need to allow devices (such as cellular user equipments, UE) to communicate directly with one another when they are under cellular network coverage. In 3GPP LTE networks (NW), this is made possible by so called “Device to Device communication (D2D), also known as Proximity Services (ProSe)” technology that allows a cellular base station (BS or eNB) to set up a direct D2D link between two UEs and allocate resources for that link.
In NSPS situations, however, the cellular network, including the eNB can get damaged or become partially or completely dysfunctional due to a disaster or emergency situation. In such situations, UEs should still be able to communicate as long as they are in the coverage area of each other or can reach each other via several D2D hops. In other words, it may be desirable that UEs can engage in local communication even when the cellular network services are in parts or completely unavailable. On the other hand, it may also be desirable that UEs get access to whatever services that are intact and still available in an NSPS situation.
Device discovery is a well known and widely used component of many existing wireless technologies, including ad hoc and cellular networks.
Examples include Bluetooth, several variants of the IEEE 802.11 standards suite such as WiFi Direct, and Flashlinq. One mechanism used by these standards is to use specially designed beacon signals that devices broadcast so that nearby devices can detect the proximity of such beacon broadcasting devices. In addition to device detection, beacon signals also allow nearby devices to detect the identity and some characteristics of the broadcasting device. Receiving devices can also use the beacon signal to estimate the channel and/or path loss to the beacon broadcasting device.
Recently, device-to-device communications as an underlay to cellular networks have been proposed as a means to take advantage of the proximity of communicating devices and at the same time to allow devices to operate in a controlled interference environment. Various device discovery mechanisms applicable for devices in cellular spectrum have also been proposed. These mechanisms make use of various forms of network assistance, such as obtaining synchronization, allocating or scheduling peer discovery resources (PDR) or tuning other parameters of the discovery process and the contents of the beacon signals.
One mechanism that addresses D2D communication both within complete network coverage and within partial network coverage, as well as outside network coverage is based on clustering, where some of the devices (UEs) act as a Cluster Head (CH) and other devices act as Slaves to the CH. In general terms, a CH node can be compared to a small range base station that,—in the absence of a cellular eNB—provides (a subset of and) similar functionality as an eNB. For example, a CH node can provide synchronization and radio resource management within its cluster and also act as a central node to create a tree topology for communication with the cluster. In addition, the CH can also provide a relaying functionality towards other clusters or towards a cellular eNB.
While clustering and cluster head based integrated ad hoc and cellular communications can meet the basic requirement on managing direct D2D communications even in the absence of the cellular NW, one technical aspect is the selection of the cluster head nodes.
One typical way to establish cluster head selection is that a given device self-nominates as cluster head and then announces itself as the cluster head. This self-nomination is based on randomness or using some other criteria. Since several devices within the proximity of each other may self-nominate as cluster heads, a mutual negotiation is typically necessary in order for one of them to remain as the single cluster head.
According to EP1978689 there is disclosed a method for cluster head selection in an ad-hoc network. According to EP1978689 a token is created by each node in a given selection process. The tokens are circulated between the nodes in the ad-hoc network. When a node receives a token it compares it to a token stored in the node. The stored token represents the best token received by the node so far. If the received token is better than the stored token it replaces the stored token in the memory of the node and the received token is also forwarded to another node in the ad-hoc network. However, if the received token is not better than the stored token it is ignored. Eventually the best token will be recorded in all the nodes and the node associated with the best token becomes cluster head.
However, there is still a need for an improved cluster head selection in a communications network.